Governmental regulations on various polluting emissions from diesel engine sources have given rise to the use of aftertreatment devices in the exhaust systems of new engines. Several such devices are well known in the art, including but not limited to particulate filters, oxidation catalysts, and nitrogen oxides adsorbers. Due to the nature of diesel engines in general, the temperature of the exhaust tends to be relatively low, especially in comparison to the exhaust temperatures of spark-ignited engines. Unfortunately, the natural exhaust temperatures of diesel engines are not well matched to the optimal operating temperatures of known aftertreatment devices. Further, during a cold engine start the aftertreatment devices are generally non-active, resulting in the discharge of non-treated exhaust gases for some period of time until the engine warms up. Even after warm-up, during some periods such as idle conditions or under low load, the exhaust temperature may again be too low for the aftertreatment devices to function properly.
Various approaches are known in the prior art for increasing the temperature of diesel exhaust gases, including post-injection of fuel in the cylinders to create an exotherm; injection of diesel fuel and air into burners in exhaust systems; electrically heated catalysts; and injection of reformed diesel fuel (reformate) into the exhaust. Each of these prior art approaches has drawbacks.
Post-injection of fuel into the cylinders has had some success in raising exhaust gas temperature in combination with an oxidation catalyst, but this approach has limitations on cold start due to a higher light off temperature of the catalyst. Also, fuel consumption can be excessive due to combustion of fuel in the cylinders, reducing the amount of hydrocarbons reaching the exhaust system to participate in the exotherm. Another drawback is the potential for wetting of the cylinder walls with fuel which can reach the crankcase and cause dilution of the engine oil supply, leading to engine wear.
Injection of diesel fuel directly into the exhaust system can be effective in heating but requires complex ignition and control systems for both fuel and air, as well as additional hardware causing increasing weight and cost of the overall engine.
Electrically heated catalyst systems require large amounts of electrical energy which are parasitic on the engine and thus reduce fuel efficiency. Further, the amount of energy and fuel consumed increases in proportion to the size of the engine.
Burning a gaseous, hydrogen-rich fuel such as reformate directly in the exhaust is a very effective means of heat transfer to the aftertreatment devices located downstream of the reformate ignition point. However, in order to get the maximum amount of heat out of the reformate, such burning should take place at about stoichiometric equivalence with oxygen. Because diesel engines have large displacements, and because exhaust flow rates above idle can be quite substantial and typically are oxygen-rich, a high flow rate of reformate may be required to produce a combustible (let alone stoichiometric) mixture of reformate and oxygen. The resulting combustion may produce heat far in excess of that required to maintain the aftertreatment devices at optimum operating temperatures.
What is needed in the art is a method and apparatus for minimizing the amount of reformate fuel required in a diesel exhaust stream to provide adequate heating for exhaust gas remediation devices.
It is a principal object of the present invention to provide supplementary heat for aftertreatment devices in a diesel exhaust stream.
It is a further object of the invention to provide such supplementary heat while minimizing the amount of fuel required to do so.